Entradasuchus
Encyclopedia
Entradasuchus is a genus
of crocodyliform
, an early member of the group including crocodilia
ns. The only known specimen was found in rocks of the Middle Jurassic
Entrada Sandstone
of Entrada Ranch, Grand County
, Utah
. Middle Jurassic terrestrial tetrapods are very rare, and Entradasuchus was the first unequivocal North American Middle Jurassic nonmarine tetrapod known from body fossils when it was described (1995).
CU-MWC
183-11, a skeleton including at least the upper surfaces of the skull, articulated armor scutes
from the neck to the tail, and some limb bones, all preserved in top (dorsal
) view. The exact composition of the specimen was unknown at the time of its description, because it is preserved in sandstone
that is harder than the calcite
that has replaced the bones, which makes preparation difficult. Additionally, much of the visible part of the skeleton is represented by mold impressions of eroded bones. The block containing the specimen weathered from canyon walls and probably came from the middle Moab Member of the Entrada Sandstone, dated to the very late middle Callovian
of the Middle Jurassic. The Moab Member of the Entrada Sandstone represents an inland dune (eolian) setting.
The skull of CU-MWC 183-11 is distinguished by the width of the posterior portion. Approximately 42 pairs of scutes ran down the midline of the back. The first pair behind the skull were square, but those on the back and tail were rectangular and had short spiny projections on their posterior outside corners. These spines inspired the name of the type species
: E. spinosus. CU-MWC 183-11 was not a large animal, with a skull only a few centimeters or inches long. This small size and the form of the skull could indicate it was a juvenile, but the fusion of the parietals
in the skull and the generally small size of animals found in settings like the upper Entrada Sandstone suggest it was an adult. Adrian Hunt and Martin Lockley
, who described Entradasuchus in 1995, classified it as Crocodyliformes incertae sedis
.
The type specimen is curated at the Museum of Western Colorado in Fruita, Colorado.
Genus
In biology, a genus is a low-level taxonomic rank used in the biological classification of living and fossil organisms, which is an example of definition by genus and differentia...
of crocodyliform
Crocodyliformes
Crocodyliformes is a clade of crurotarsan archosaurs, the group often traditionally referred to as "crocodilians."In 1988, Michael J. Benton and James M. Clark argued that all traditional names for well-known groups of animals should be restricted to their crown clades, that is, used only for...
, an early member of the group including crocodilia
Crocodilia
Crocodilia is an order of large reptiles that appeared about 84 million years ago in the late Cretaceous Period . They are the closest living relatives of birds, as the two groups are the only known survivors of the Archosauria...
ns. The only known specimen was found in rocks of the Middle Jurassic
Middle Jurassic
The Middle Jurassic is the second epoch of the Jurassic Period. It lasted from 176-161 million years ago. In European lithostratigraphy, rocks of this Middle Jurassic age are called the Dogger....
Entrada Sandstone
Entrada Sandstone
The Entrada Sandstone is a formation in the San Rafael Group that is spread across the U.S. states of Wyoming, Colorado, northwest New Mexico, northeast Arizona and southeast Utah...
of Entrada Ranch, Grand County
Grand County, Utah
Grand County is a county located in the U.S. state of Utah. As of 2000 the population was 8,485, and by 2005 had been estimated at 8,743. It was named for the Colorado River, which at the time of statehood was known as the Grand River. Its county seat and largest city is Moab.-Geography:According...
, Utah
Utah
Utah is a state in the Western United States. It was the 45th state to join the Union, on January 4, 1896. Approximately 80% of Utah's 2,763,885 people live along the Wasatch Front, centering on Salt Lake City. This leaves vast expanses of the state nearly uninhabited, making the population the...
. Middle Jurassic terrestrial tetrapods are very rare, and Entradasuchus was the first unequivocal North American Middle Jurassic nonmarine tetrapod known from body fossils when it was described (1995).
Description
Entradasuchus is based onHolotype
A holotype is a single physical example of an organism, known to have been used when the species was formally described. It is either the single such physical example or one of several such, but explicitly designated as the holotype...
CU-MWC
University of Colorado Denver
The University of Colorado Denver, shortened as CU Denver, UC Denver, or UCD, is a public university in the United States state of Colorado. It is one of three schools of the University of Colorado system. The university has two campuses — one in downtown Denver at the Auraria Campus, and the other...
183-11, a skeleton including at least the upper surfaces of the skull, articulated armor scutes
Osteoderm
Osteoderms are bony deposits forming scales, plates or other structures in the dermal layers of the skin. Osteoderms are found in many groups of extant and extinct reptiles, including lizards, various groups of dinosaurs , crocodilians, phytosaurs, aetosaurs, placodonts, and hupehsuchians...
from the neck to the tail, and some limb bones, all preserved in top (dorsal
Anatomical terms of location
Standard anatomical terms of location are designations employed in science that deal with the anatomy of animals to avoid ambiguities that might otherwise arise. They are not language-specific, and thus require no translation...
) view. The exact composition of the specimen was unknown at the time of its description, because it is preserved in sandstone
Sandstone
Sandstone is a sedimentary rock composed mainly of sand-sized minerals or rock grains.Most sandstone is composed of quartz and/or feldspar because these are the most common minerals in the Earth's crust. Like sand, sandstone may be any colour, but the most common colours are tan, brown, yellow,...
that is harder than the calcite
Calcite
Calcite is a carbonate mineral and the most stable polymorph of calcium carbonate . The other polymorphs are the minerals aragonite and vaterite. Aragonite will change to calcite at 380-470°C, and vaterite is even less stable.-Properties:...
that has replaced the bones, which makes preparation difficult. Additionally, much of the visible part of the skeleton is represented by mold impressions of eroded bones. The block containing the specimen weathered from canyon walls and probably came from the middle Moab Member of the Entrada Sandstone, dated to the very late middle Callovian
Callovian
In the geologic timescale, the Callovian is an age or stage in the Middle Jurassic, lasting between 164.7 ± 4.0 Ma and 161.2 ± 4.0 Ma. It is the last stage of the Middle Jurassic, following the Bathonian and preceding the Oxfordian....
of the Middle Jurassic. The Moab Member of the Entrada Sandstone represents an inland dune (eolian) setting.
The skull of CU-MWC 183-11 is distinguished by the width of the posterior portion. Approximately 42 pairs of scutes ran down the midline of the back. The first pair behind the skull were square, but those on the back and tail were rectangular and had short spiny projections on their posterior outside corners. These spines inspired the name of the type species
Type species
In biological nomenclature, a type species is both a concept and a practical system which is used in the classification and nomenclature of animals and plants. The value of a "type species" lies in the fact that it makes clear what is meant by a particular genus name. A type species is the species...
: E. spinosus. CU-MWC 183-11 was not a large animal, with a skull only a few centimeters or inches long. This small size and the form of the skull could indicate it was a juvenile, but the fusion of the parietals
Parietal bone
The parietal bones are bones in the human skull which, when joined together, form the sides and roof of the cranium. Each bone is roughly quadrilateral in form, and has two surfaces, four borders, and four angles. It is named from the Latin pariet-, wall....
in the skull and the generally small size of animals found in settings like the upper Entrada Sandstone suggest it was an adult. Adrian Hunt and Martin Lockley
Martin Lockley
Dr. Martin G. Lockley is an English paleontologist specializing in dinosaur tracks. He is a Professor of Geology at the University of Colorado at Denver, in the United States, and founder and director of the Dinosaur Trackers Research Group.-Biography:...
, who described Entradasuchus in 1995, classified it as Crocodyliformes incertae sedis
Incertae sedis
, is a term used to define a taxonomic group where its broader relationships are unknown or undefined. Uncertainty at specific taxonomic levels is attributed by , , and similar terms.-Examples:*The fossil plant Paradinandra suecica could not be assigned to any...
.
The type specimen is curated at the Museum of Western Colorado in Fruita, Colorado.